1. Field of the Invention
The present invention relates to an image display apparatus that includes spontaneous light-emitting diodes. More particularly, the present invention relates to an image display apparatus of an active-matrix type including driver elements having a current passage region made of amorphous silicon.
2. Description of the Related Art
Currently, image display apparatuses made with organic light emitting diodes (hereinafter, “organic LEDs”) are attracting attention as a replacement for the liquid crystal display devices. The image display apparatus including organic LEDs requires no back light because it is spontaneously emissive, and has excellent response speed, contrast, and visibility. The image display apparatus including organic LEDs has an advantage in production costs because it has a relatively simple structure.
FIG. 8 is an equivalent circuit diagram illustrating a structure of a conventional image display apparatus of the active-matrix type including organic LEDs. As illustrated in FIG. 8, the conventional image display apparatus includes an organic LED 101, and a power line 107 connected to a cathode of the organic LED 101 to supply a current to the organic LED 101 under the control of a power line driver circuit 110. The organic LED 101 is connected to a thin film transistor 102, serving as a driver element, that controls the current flowing in the organic LED 101. The thin film transistor 102 is controlled for drive state when a certain gate potential is given from a data line 104 and a scan line 105 via a thin film transistor 103 thereto. This control allows determination of the current flowing in the organic LED 101. The organic LED 101 emits a light of specific intensity in response to the current flowing therein for image display (see for example, Japanese Patent Application Laid-Open No. H8-234683).
The thin film transistor 102 has a drive state-based mobility and accordingly has a function of controlling the current flowing in the organic LED 101 based on the gate potential applied. The proposed structure of the thin film transistor 102 includes a channel layer composed of either polysilicon or amorphous silicon.
The thin film transistor including the channel layer composed of polysilicon can achieve an increased mobility but has a problem that the particle diameter of polysilicon forming the channel layer can not be controlled. The mobility in the thin film transistor of polysilicon suffers the influence of the particle diameter of the polysilicon forming the channel layer. Therefore, the thin film transistor 102 may differ in mobility per pixel if the particle diameter is hardly controlled. For example, a monochromic display over the whole screen requires an equal gate voltage to be applied to the thin film transistors 102 configuring respective pixels. In this case, the thin film transistor of polysilicon is hardly controlled for the particle diameter and thus differs in mobility per pixel, resulting in differences in current flowing in the organic LED 101. Because the organic LED 101 is a current-driven light emitting diode, brightness per pixel fluctuates depending on the current flowing therein, and it is almost impossible to achieve the monochromic display.
To the contrary, the thin film transistor including the channel layer composed of amorphous silicon requires no control for particle diameter. Therefore, an individual thin film transistor arranged per pixel has no problem on difference in mobility. Accordingly, the thin film transistor 102 employed as the driver element for the organic LED is preferable to include the channel layer composed of amorphous silicon. The use of the thin film transistor thus structured allows almost uniform current to flow in the organic LED individually.
The use of the thin film transistor including the channel layer composed of amorphous silicon as the driver element, however, causes a problem because the conventional image display apparatus illustrated in FIG. 8 is difficult to perform image display over a long time. It is known that the threshold voltage varies in the thin film transistor of amorphous silicon when a current flows in the channel layer for a long time. This is because the current flowing in the channel layer varies according to the fluctuation of the threshold voltage even though a constant gate voltage is applied continuously.
FIG. 9 is a graph of variations in fluctuation values of the threshold voltage resulted from continuous flow of a constant current in the channel layer of the thin film transistor including the channel layer of amorphous silicon. In the graph of FIG. 9, the current flowing in the channel layer is controlled in such a manner that a light emitted by a general organic LED is of a brightness of 150 cd/m2.
As is obvious from FIG. 9, fluctuation of almost 1 volt occurs in the threshold voltage after about 100 hours elapsed, and fluctuation of more than 2 volts occurs in the threshold voltage after 2,000 hours elapsed. In general, the image display apparatus including the organic LEDs is required to have a constant brightness for 20,000 hours continuously. Accordingly, it is not desirable that the threshold voltage fluctuates greatly in a short time.
An actual image display apparatus including thin film transistors of amorphous silicon employed as driver elements therefore requires a voltage compensator circuit arranged per pixel in addition to the structure illustrated in FIG. 8. In a specified structure to achieve stable image display, the voltage compensator circuit applies a potential to compensate the threshold voltage for fluctuation components to the gate electrode of the thin film transistor 102 in addition to the potential supplied from the data line 104. Such voltage compensator circuit includes three or four thin film transistors per pixel and additionally requires a place to arrange the voltage compensator circuit on the substrate together with the organic LEDs. Therefore, a new problem arises because the organic LEDs can not be arranged at a high density and a high-resolution image is hardly displayed.
The thin film transistor of amorphous silicon has a problem that it has a low mobility originally. When a conventional organic LED is employed as a light emitting diode, it requires a certain amount of current to achieve a sufficient brightness. Accordingly, an expanded channel width is required to supply such current in the organic LED, resulting in an increase in an area occupied by the thin film transistor serving as the driver element. As a result, a problem arises because an arrangement density of the organic LEDs is lowered and a high-resolution image is hardly displayed.